1. Field of the Invention
The present invention relates to a ceramic catalytic converter disposed in an exhaust passage of an engine for a vehicle.
2. Related Arts
In general, a ceramic catalytic converter is provided in an exhaust passage of an engine for a vehicle for purifying exhaust gas. Such a ceramic catalytic converter typically has a ceramic catalytic support carrying a catalyst layer on a surface thereof, a metallic cylindrical casing for holding the catalytic support therein, and a holding member disposed on a circumferential surface of the catalytic support in a gap between the catalytic support and the cylindrical casing.
The ceramic catalytic support is conventionally made of a cordierite-system ceramic material (2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2) having a low thermal expansion coefficient and this material generally has been utilized for the ceramic catalytic support for years. The catalyst layer formed on the surface of the catalytic support includes noble metals such as platinum (Pt), rhodium (Rh), lead (Pd) and the like for converting undesirable ingredients such as carbon monoxide (CO), hydrocarbon (HC), oxides of nitrogen and the like included in the exhaust gas from the engine into harmless gases and water.
The holding member is utilized to prevent damage to the ceramic catalytic support which does not have a sufficient mechanical strength. The holding member generally has an elastic body made of various kinds of ceramic fibers. For example, as shown in FIG. 20A, a mat-like holding member 9 disclosed in JP-A-7-102961 is made of ceramic fibers which have a length of 30 mm or more, and are oriented generally in one direction. The holding member 9 is disposed on a ceramic catalytic support 12 (shown in FIG. 20B) so that the orientation of the ceramic fibers is generally parallel to an axial direction of the ceramic catalytic support 12.
However, the ceramic fibers forming the holding member 9 are oriented in one direction and hardly intertwined with each other. Therefore, when tensile force is applied to the holding member 9 in a perpendicular direction with respect to the orientation of the ceramic fibers, the ceramic fibers are liable to be separated from each other. A shearing force applied on the holding member 9 in a direction parallel to the elongating direction of the ceramic fibers also can easily cause the separation of the ceramic fibers. The separation of the ceramic fibers loosens the holding member 9, resulting in a decrease of the holding force of the holding member 9 for holding the ceramic catalytic support 12 in the cylindrical casing. As a result, damage to the catalytic support 12 may occur.
The above-mentioned problem is more likely to arise in a high temperature atmosphere. In the high temperature atmosphere, the thermal expansion coefficients of the ceramic catalytic support 12 and the cylindrical casing are different from each other, so that the gap between the ceramic catalytic support 12 and the cylindrical casing increases. Accordingly, the ceramic catalytic support 12 is liable to rattle within the cylindrical casing. In addition, when manufacturing the holding member 9, it is technically difficult to arrange the ceramic fibers during lamination so that they are oriented in one direction.